The present invention relates to substrate processing. More specifically, the present invention relates to an apparatus and method for the cleaning of a substrate processing system's processing chamber.
One of the primary steps in the fabrication of modern semiconductor devices is the formation of a layer, such as a metal silicide layer like tungsten silicide (WSi.sub.x), on a substrate. As is well known, such a layer can be deposited by chemical vapor deposition (CVD). In a conventional thermal CVD process, reactive gases are supplied to the substrate surface where heat-induced chemical reactions take place to form the desired film over the surface of the substrate being processed. In a conventional plasma-enhanced CVD (PECVD) process, a controlled plasma is formed using radio frequency (RF) energy or microwave energy to decompose and/or energize reactive species in reactant gases to produce the desired film.
One problem that arises during such CVD processes is that unwanted deposition occurs on some or all of the processing chamber's interior surfaces, leading to potentially high maintenance costs. With CVD of a desired film onto a substrate, the deposition of undesired residues can occur on any surface including the heater and process kit parts of the apparatus, because the reactive gases can diffuse to most parts of the processing chamber, even between cracks and around comers. During subsequent substrate depositions, these residues can accelerate until a continuous metal silicide film is grown on the heater and/or these other parts. Over time, failure to clean the residue from the CVD apparatus often degrades process yield. When excess deposition starts to interfere with the CVD system's performance, the heater and other process kit parts (such as the shadow ring and gas distribution faceplate) can be replaced to remove unwanted accumulations thereon. Depending on which and how many parts need replacing and the frequency of the replacement, the cost of maintaining the substrate processing system can become very high. Moreover, such maintenance adversely affects throughput of the CVD system.
In these CVD processes, cleaning of the processing chamber is regularly performed to remove such unwanted residues from the chamber walls, heater, and other process kit parts. Commonly performed between deposition steps for every substrate (or every n substrates), in situ cleaning procedures using one or more cleaning (i.e., etchant) gases are performed to remove the unwanted residual material accumulated during the deposition process. Common cleaning techniques include thermal, RF plasma, and microwave plasma techniques that promote dissociation of the reactant gases to generate highly reactive species (such as halogen radicals (highly reactive, single halogen atoms)), and are often referred to as "dry" cleaning techniques. In these techniques, the halogen radicals react with and etch away the unwanted residues from the chamber walls and other surfaces. However, the etching gases useful for cleaning the chamber are often corrosive and can attack not only the residues being removed, but also the materials which make up the chamber, heater, and process kit components. This is particularly true for certain cleaning gases used in conjunction with metal CVD processes.
Such is the case for thermal processes using chlorine trifluoride (ClF.sub.3). In such an in situ thermal cleaning process, ClF.sub.3 is flowed into the processing chamber being cleaned. Thermal energy (i.e., heat) is then applied to the ClF.sub.3 to generate fluorine radicals (i.e., single fluorine atoms, denoted F*). When cleaning a processing chamber in which a tungsten silicide (WSi.sub.x) deposition process has been performed, these fluorine radicals combine with the tungsten (W) and silicon (Si) residues to form tungsten fluoride (WF.sub.y) and silicon fluoride (SiF.sub.z), which, being volatile products, can then be exhausted from the processing chamber.
The advantages provided by a thermal process using ClF.sub.3 include the need for few modifications to the substrate processing system to take advantage of such a technique. Because the ClF.sub.3 is easily dissociated, a simple thermal method is all that is required to generate the required fluorine radicals, rather than the more-complex RF plasma-based cleaning techniques. Moreover, a thermal technique causes minimal damage to the process kit. However, this technique also has certain disadvantages. Certain of the gases generated as by-products of this process (e.g., chlorine and fluorine) are highly corrosive and environmentally undesirable. Thus, complex exhaust hardware is required to abate these dangerous gases. Due to their corrosive nature, frequent maintenance of the exhaust system is required, as is the replacement of critical elements of such an exhaust system. This adds to the cost of producing and maintaining such a system, and can adversely affect system up-time (i.e., throughput).
A second alternative for in situ cleaning of processing chambers is the use of an RF plasma. An RF plasma cleaning process could use nitrogen trifluoride (NF.sub.3), for example, because such a technique is capable of imparting the high energies required to dissociate a more stable compound such as NF.sub.3. First, NF.sub.3 is flowed into the processing chamber being cleaned. RF energy is then applied (e.g., via the substrate processing system's capacitively-coupled electrodes), thus generating the fluorine radicals (F*) which remove the unwanted residues from the processing chamber's components. A frequency of 13.56 megahertz (MHz) is commonly used to excite the plasma. As above, when cleaning a processing chamber in which a tungsten silicide deposition process has been performed, the fluorine radicals combine with the tungsten and silicon residues to form tungsten fluoride and silicon fluoride, which, being volatile products, can then be exhausted from the processing chamber.
This technique is well-known, and so is well-understood. However, this technique also has certain disadvantages. In contrast to the thermal technique discussed above, an RF plasma process using NF.sub.3 reduces both environmental and maintainability concerns, although fluorine-containing by-products are still generated by such a technique. Although ClF.sub.3 (or compounds having similar dissociation energies) could be used in such an RF plasma cleaning process, the aforementioned problems related to the use of ClF.sub.3 would again be encountered.
Chief among these disadvantages is the damage done to the process kit's components by such a cleaning technique. RF plasma cleaning causes ion bombardment of the metallic processing chamber components, causing physical damage to components such as the interior chamber walls. Therefore, such in situ cleaning may make it difficult to effectively clean residues without also eventually damaging the heater and other chamber components. Thus, maintaining chamber performance may result in damage to expensive consumable items, resulting in the need for frequent replacement. Also, inert gases such as argon (Ar) are often added in order to enhance the striking of the RF plasma. Because such elements often have greater atomic masses (i.e., are larger and heavier) than the constituent elements in the cleaning gas (e.g., N and F), the presence of such inert gases in the RF plasma can exacerbate the ion bombardment problem. The physical damage caused by ion bombardment also presents the possibility of particle generation. Other concerns are the sensitivity of such techniques to the timing of the cleaning process and the fact that areas not in contact with the plasma may not be properly cleaned.
Another alternative is the use of microwave frequencies to create the requisite plasma and dissociate the cleaning gas. In a microwave plasma technique, the plasma is struck by applying microwave energy to a cleaning gas such as NF.sub.3, thus generating fluorine radicals. These radicals clean the interior of the processing chamber, as before. A microwave frequency of 2450 MHz is commonly used to excite the plasma. Advantages of such a technique include compatibility with pre-existing RF plasma cleaning systems and the highly efficient generation of halogen radicals. The high breakdown efficiency provided by a microwave plasma technique (at least 50%, but more likely on the order of about 99%) results in a higher etch rate (on the order of about 2 .mu.m/min in the case of tungsten silicide residues) than is obtained with a capacitive RF plasma (which has a relatively low breakdown efficiency of between about 15% and 30%). This translates into faster and more thorough cleaning of the processing chamber's interior surfaces. Also, given the high percentage of fluorine converted into fluorine radicals (and their subsequent reaction with residues), the amount of fluorine produced by such systems is reduced, lessening concerns over the corrosive nature of the exhaust gases and attendant environmental effects. Some substrate systems must be modified significantly to employ such a technique. The problem of ion bombardment may also be encountered with such techniques.
As an alternative to an in situ microwave plasma cleaning technique, a separate microwave plasma system may be attached to the substrate processing system. As with the in situ microwave plasma technique, the high breakdown efficiency of a microwave plasma technique results in a high etch rate, providing the aforementioned benefits. However, unlike their in situ counterparts, remote microwave plasma generation systems provide radicals without subjecting the processing chamber's components to a plasma of any sort during cleaning operations. A remote microwave plasma cleaning technique can therefore more gently, efficiently, and adequately clean residues within the processing chamber without the physical damage to the gas distribution manifold, the inside chamber walls, and other processing chamber components that may be experienced with in situ techniques. Present systems can be easily modified to take advantage of such remote microwave plasma techniques because such microwave plasma systems are fairly self-contained.
One common problem the aforementioned cleaning techniques encounter is the differences in cleaning (etching) rates experienced by the various interior surfaces of the processing chamber being cleaned. As noted, in either some surfaces being over-etched, or a failure to remove the residues from others. A primary source of this disparity is variation in the surface temperature of the processing chamber components being cleaned. As is well known, the hotter the material, the faster that material will etch, all other chamber parameters remaining constant. Certain substrate processing systems (referred to herein as "hot-walled" systems) maintain the processing chamber's interior surfaces at a constant temperature by liquid heating (or cooling) of certain processing chamber components, most notably the chamber's walls. In such systems, cleaning operations tend to proceed more evenly from one surface to another because the etch rates remain fairly constant between the various surfaces of the processing chamber. In systems which provide no such control over surface temperatures (referred to herein as "cold-walled" systems), etch rates can differ significantly, with residues accumulated on hot surfaces (e.g., heated substrate pedestals) etching quickly and those accumulated on cold surfaces (e.g., chamber walls) etching more slowly. This results in either the incomplete cleaning of the cold surfaces or over-etching of the hot surfaces.
Because of problems related to the incomplete cleaning of certain processing chamber components, and the fact that few processing chamber components can be cleaned completely without causing unacceptable damage to other processing chamber components, additional cleaning procedures must be performed. Such procedures (often referred to as a "wet" cleaning) involve opening the processing chamber and physically wiping the entire processing chamber--including the chamber walls, exhaust and other areas having accumulated residue--with a special cloth and cleaning fluids, and so is carried out less frequently than in situ cleaning processes. Without frequent cleaning procedures (both in situ and wet cleanings), impurities from the residues in the CVD apparatus can migrate onto the substrate and cause device damage.
It can therefore be seen that the proper cleaning of a CVD system is essential to the reliable operation of the substrate processing system, and maintaining acceptable device yield and system throughput. Thus, given the foregoing, a microwave plasma cleaning system that permits the efficient cleaning of a processing chamber is desirable. It is also desirable to provide efficient generation of reactive radicals, while minimizing damage to the processing chamber's components that are exposed to the cleaning process. A cleaning technique which reduces the frequency with which wet cleaning operations must be performed is also desirable. Finally, such a cleaning technique should clean the processing chamber's interior surfaces evenly, regardless of whether a hot-walled or cold-walled system is being cleaned.